1. Field of the invention
The present invention relates to a nonwoven fabric comprising synthetic organic fiber, more specifically to such nonwoven fabric for reinforcement of resins which can give molded articles with excellent impact resistance, and also to moldable sheets comprising said nonwoven fabric impregnated with resin compositions.
2. Description of the prior art
Moldable sheets reinforced with reinforcing materials, represented by SMC (sheet molding compound), have been produced by a process which comprises:
consolidating a resin composition comprising unsaturated polyester resin and a filler, color, mold release, curing agent, thickener and the like, with short-cut chips of glass fiber roving yarn (known as "glass fiber strands") to form an endless sheet, PA1 covering both surfaces of the sheet With polyethylene film or the like, PA1 compressing the sheets to impregnate the resin composition into spaces between the glass fiber strands and to deaerate the sheet, and PA1 ageing the sheet at an appropriate temperature. PA1 (1) using organic fibers, particularly those having high strength and high elastic modulas, as reinforcement fiber, PA1 (2) dispersing the single filaments relatively uniformly, PA1 (3) using the organic fibers having large single-filament fineness, and PA1 (4) using the organic fibers in the form of nonwoven fabric.
Glass fiber has generally been used for reinforcing resins, since it is excellent in mechanical properties such as fiber strength and rigidity, resistance to heat and dimensional stability, as well as in processability and the like.
The most serious drawback of resinous shaped articles reinforced with fiber (hereinafter referred to as FRP) in which glass fiber is used is inadequate impact resistance. If an FRP has a high impact strength, in particular high falling ball impact strength, the shaped article will, when given a shock, not readily generate cracks or whitening on its surface and thus maintain its high quality and neat appearance. While high falling ball impact resistance is therefore an indispensable requirement for shell plating of automobiles, railroad cars, ships, etc., as well as for pipes, bathtubs and the like, the use of glass fiber strands can never meet the requirement in practice. Accordingly, there has been strongly desired a technique that would bring a leap in improving falling ball impact strength in the field of FRP.
The present inventors had studied to improve the falling ball strength of FRP and found that the object can be achieved by:
The present inventors have further studied based on the findings (1) through (4) and completed the invention.
Japanese Patent Application Laid-open No. 42952/1988 discloses a nonwoven fabric used for reinforcing resins, which comprises non-glass-fiber staple fiber, the staple fiber being present as a mixture of one group of fiber opened into single filaments and the other group of fiber comprising unopened bundles comprising a plurality of single filaments laid parallel with one another, the two groups being bonded with each other. The patent application also describes that the nonwoven fabric having the above construction gives an FRP product, with a fiber content less than half that in the case where glass fiber chopped strand mat is used, having both high strength and high elastic modulus. However, the patent application does not define the combined state of opened single filaments and unopened strands (bundles). The tensile strength, flexural strength, impact strength and the like of an FRP product varies to a large extent depending on the fiber fineness, state of single filaments gathered and distribution of the gathered-filament bundles. Simple incorporation of opened single filaments and unopened bundles will therefore not always give a good FRP. Furthermore, the invention utilises an adhesive fiber for bonding the groups of fiber. Adhesive fiber must be incorporated in a large amount that can assure firm bonding, which however decreases the ratio incorporated of the reinforcing fiber, thereby decreasing reinforcement effect. The use of an adhesive fiber has another drawback in that the bonds between single filaments and filament bundles, and between bundles themselves are, during formation of molded articles, difficult to release in practice, and that hence such FRP has poor fluidity for molding deep-drawn articles. The problem has also been solved by the present invention.